The present invention relates to adaptive optics, and more particularly, to a hill-climbing adaptive optics system having a variable aperture for mitigating 2.pi.N ambiguity in the control system signals.
The intensity of radiation incident onto a remote target is often compromised by irregularities inherent in the phase distribution of the wavefront of the beam of radiation transmitted to the target. Ideally, the radiation incident onto a remote target should have a waterfront distribution which is near diffraction limited for many applications. However, distortions of the wavefront distributions can result from inhomogenuities within the gain medium of the laser, atmospheric turbulence and thermal blooming along the path of the radiation transmitted to the target, mechanical vibrations and thermally induced distortions of the optical components defining the laser system and by other means well known in the art. To insure a near diffraction limited beam incident onto a remote target, a dynamic active feedback control system is required for providing phase-induced corrections to the wavefront distribution of the beam to compensate the aforesaid distortions. Such feedback control systems are well known in the art.
Angelbeck et al in U.S. Pat. No. 4,091,274 discloses an active laser mirror system for achieving near diffraction limited transmission of laser radiation by actively controlling a reflective surface in the system to induce phase corrections within the beam for compensating phase distortions introduced by inhomogenuities within the path of the transmitted beam. The system incorporates a feedback loop including actuators disposed on a mirror for inducing phase corrections into the radiation wavefront. O'Meara in U.S. Pat. No. 3,731,103 discloses a laser system wherein adaptive phase control is used to establish an in-phase condition for a transmitted beam at a remotely located target. A plurality of subaperture areas of the transmitted beam are phase modulated at distinct modulation frequencies. Amplitude fluctuations in a received beam, at the modulation frequencies, are indicative of phase distortions, from the diffraction limited distribution, in the transmitted beam. The phase control system adjusts the relative phase of the subaperture areas of the transmitted beam to minimize the amplitude fluctuations in the received beam to establish an in-phase condition of the wavefront distribution of the transmitted beam on the target. See also U.S. Pat. Nos. 3,727,223 and 3,764,213.
Freiberg in copending U.S. patent application Ser. No. 911,259, which is held by a common assignee with the present application, discloses a resonator feedback system wherein the wavefront distribution of the output beam of a laser is controlled by adaptive optic elements within the cavity defining the laser resonator. A servo control system provides multi-correction signals to the adaptive optic elements to induce phase corrections to the radiation within the resonator to provide a near diffraction limited output beam.
Common to most prior art adaptive optics systems is the requirement that the system wavefront error be brought to within .+-.2.pi. radians of reference before the initial turn-on of a multidither controller to drive the system to a high quality null. In a local loop multidither adaptive optics system, the requirement for pre-alignment to eliminate the 2.pi. ambiguities necessitates the use of a figure sensor to measure the optical surface figure of the corrector mirror. This added sensor substantially increases the cost and complexity of the adaptive optics system. In a typical adaptive optics system, the 2.pi.N ambiguity results from the utilization of a diaphragm having a small aperture, typically with a diameter approximating one-tenth the diameter of the Airy disk, at the Fourier transform plane of a focusing mirror within the feedback loop that permits only a small portion of the focused radiation to enter a detector. When sections of the wavefront distribution differ by 2.pi.N in optical phase, local non-global maxima occur in the aperture and the control system converges on these non-global local maxima rather than on the global maxima resulting in a 2.pi.N in ambiguity in the convergence. Thus, in essentially all of the prior art systems utilizing continuous surface deformable mirrors as the wavefront correction element, correctable phase error has to be reduced to less than 2.pi. before the control loop is activated to insure convergence at a global maxima.